Image sensors convert light signals into electrical signals (e.g. analog signals). In some circumstances, it may be necessary to convert an analog signal into a digital signal, in order to perform several-stepped image processing procedures for providing an image signal to a display device (e.g. a liquid crystal display or similar devices). Often, an analog-to-digital converter (hereinafter referred to as an ADC) may be used.
In general, the more bits that are output from the ADC, the higher the resolution of the ADC. Accordingly, when a relatively finely divided analog signal is input into the ADC, it may be desirable for the ADC to output a relatively high data digital signal. An ADC may include a flash ADC, an integration ADC, a staircase ADC, a tracking ADC, and/or similar devices.
An integration ADC may be adapted to precisely convert analog signals into digital signals. The integration ADC may be configured with an integrator, a comparator, and/or a counter. An integration ADC may count clock pulses during a time duration proportional to the voltage of an input analog signal and output the count as a digital signal. The duration proportional to the voltage of the input analog signal may be obtained using the integrator. A reference voltage and a ramp voltage generated in a ramp voltage generator may be applied to the comparator, and the counter may latch the count at an instant when the ramp voltage exceeds the reference voltage. The latched count may be provided as a digital value. The ramp voltage generator may include a single slope integrator configured with capacitors and resistors.
FIG. 1 is a graph illustrating outputs of an ADC including a single slope integrator, in accordance with the related art. A vertical axis denotes the output of a single slope ADC including a single slope integrator and a horizontal axis denotes an input voltage to the ADC. The single slope ADC may not cause any problems when processing a small number of pixels. However, as the number of pixels becomes larger, the single slope ADC may need higher processing speeds. A two-step slope ADC may be used instead of the single slope ADC. However, the two-step slope ADC requires a two-step ramp signal generator configured to generate at least two stepped ramp signals in order to convert an analog signal into a digital signal.
FIG. 2 illustrates a waveform diagram of ramp signals with a two-step slope, in accordance with the related art. In a two-step slope ADC, a maximum value of a first stepped ramp signal Vramp(n−1) may often be different from an initial value of a second stepped ramp signal Vramp(n). Therefore, when the ADC produces a digital signal, code errors corresponding to the digital signal may occur (e.g. a missing code may be generated).